1. Field of the Invention
The present invention relates to touchpads.
2. Background Art
There is a constant need for improved human-machine interfaces in small portable devices such as laptop and palm-top computers, portable phones, portable entertainment players, video games, personal digital assistants, remote controls, and the like. This need is driven by increasing functionality and the convergence of functions into single devices. Cell phones, for example, now often include a camera, personal digital assistant, video and audio download and playback, web browsing, and multiple types of messaging.
Touchpads are a very versatile class of user input devices. When used with a graphical user interface (GUI), touchpads can be used to input relative motion, absolute position selection, scrolling, text input, and the like. Defined by the paradigm of rectangular coordinates, most touchpads are by default rectangular. Computer screens are rectangular and laptops themselves are rectangular. Thus, the operating principle of most touchpads is also rectangular.
One example of a rectangular touchpad is referred to as a four-wire rectangular resistive touchpad. In this device, two resistive plates are separated by a small air gap. One plate, referred to as the horizontal plate, has vertical highly conductive traces at each end. The other plate, referred to as the vertical plate, has horizontal highly conductive traces at each end. These plates are rectangular and the traces are orthogonal. Each plate is energized in turn by placing a voltage across its end traces so that the voltage across the plate drops linearly from one end to other. While one plate is energized, the non-energized plate is connected to a voltage measuring circuit. When a user touch brings the two plates into contact, the measured voltage is proportional to the touched distance along the energized plate. Both electrically and logically, this four-wire touchpad sets up a rectangular coordinate system. Most non-rectangular plates are not possible in this paradigm, since the position-indicating voltages would be distributed in patterns which would be difficult to detect and resolve.
The traditional rectangular paradigm has several disadvantages. The first is that many tasks that may be performed using a touchpad are not necessarily rectangular. A typical use, moving a cursor between two points, could just as easily be viewed as native to a polar coordinate system.
A second shortcoming is aesthetic. Small handheld devices are often viewed as fashion accessories. In a competitive market, designers seek to differentiate their products by appearance and shapes other than rectangular may be more attractive.
A third reason to consider a non-rectangular touchpad is a lack of space. Non-rectangular shapes may be easier to arrange within an array of other input or output components.
A fourth reason is that, on small touchpads, corners are difficult to reach. In contrast, a small round touchpad has all edges equally accessible.
What is needed is a resistive touchpad configuration that can be easily and inexpensively adapted to a wide variety of touchpad applications, including non-rectangular applications.